A copper alloy used in electronic components such as a lead frame, connector, pin, terminal, relay and switch is required to satisfy both high-strength and high-electrical conductivity (or thermal conductivity) as a basic characteristic. In recent years, as high-integration and reduction in size and thickness of an electronic component have been rapidly advancing, requirements for copper alloys used in these electronic components have been sophisticated more than ever.
However, the characteristics of copper alloys as well as other alloys are affected by composition elements and structures, and condition of heat-treatment. In addition, the predictability of the effect caused by a subtle change in composition elements or condition of heat-treatment on the characteristics of the alloys is generally very low. Therefore, it has been very difficult to develop a novel copper alloy satisfying the continuously increasing requirements.
In recent years, with consideration to high-strength and high-electrical conductivity, the usage of age hardening copper alloys in electronic components has been increasing, replacing traditional solid-solution hardening copper alloys as typified by phosphor bronze and brass. In the age hardening copper alloys, the age hardening of supersaturated solid solution, which underwent solution treatment beforehand, disperses fine precipitates uniformly, thereby increasing the strength of the alloys. At the same time, it also reduces the amount of solved elements contained in the copper, thereby increasing the electrical conductivity. For this reason, it provides materials having excellent mechanical characteristics such as strength and stiffness, as well as high electrical and thermal conductivity.
Among age hardening copper alloys, Cu—Ni—Si copper alloys are typical copper alloys having relatively high electrical conductivity, strength, good stress relaxation characteristic and bending workability, and therefore they are among the alloys that have been actively developed in the industry in these days. In these copper alloys, fine particles of Ni—Si intermetallic compounds are precipitated in copper matrix, thereby increasing strength and electrical conductivity.
In general, the precipitation of Ni—Si intermetallic compounds, which contributes to strengthening, is composed of stoichiometric composition. For example, Japanese patent laid-open publication No. 2001-207229 discloses a way of achieving good electrical conductivity by bringing the mass ratio of Ni and Si in an alloy close to the concentration of the intermetallic compound, Ni2Si, namely, by adjusting the mass ratio of Ni and Si such that the ratio Ni/Si becomes from 3 to 7.
Further, Japanese patent publication No. 3510469 states that, similar to Ni, Co forms a compound with Si, thereby increasing strength, and Cu—Co—Si alloys, when age-hardening, have slightly better strength and electrical conductivity than Cu—Ni—Si alloys. Further, it also states that, where acceptable in cost, Cu—Co—Si and Cu—Ni—Co—Si alloys may be also selectable,
Further, Japanese patent publication No. 2572042 mentions Co as an example of a silicide forming elements and impurities which give no adverse effect on properties of copper alloys. It also states that such element, if existed in the alloy, should be contained by replacing the equivalent amount of Ni, and may be contained in the effective amount equal to or less than about 1%.
Further, Japanese patent publication No. 3049137 states that the addition of Co is effective in the structure control of Ni—Si copper alloy manufacture, in which the amount of S and other manufacturing conditions are controlled. It also states that a very small amount of Cr addition is effective in the improvement of hot workability, and the amount of Cr should be no larger than 0.1% by weight.
However, since Co is more expensive than Ni as stated in the aforementioned document, and thereby has the drawback in practical use. Therefore, no or few meticulous studies have been conducted on Cu—Ni—Si alloys using Co as an additive element in the past. In addition, it has been believed that, similar to Ni, Co forms compounds with Si, and slightly increases mechanical strength and electrical conductivity by replacing Ni. However, it has never been conceived that Co dramatically improves characteristics of alloys. In addition, it have been strongly believed that the addition of Cr contributes to strengthening only when it is precipitated by itself, and it has not been recognized as an element which dramatically improves electrical conductivity without adversely affecting strength.